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United States Patent |
5,546,294
|
Schutten
,   et al.
|
August 13, 1996
|
Resonant converter with wide load range
Abstract
Low-power auxiliary circuitry is added to a resonant converter for
providing high efficiency operation, low EMI, and tight output voltage
control over a wide load range. There is an auxiliary circuit
corresponding to each half-bridge connection of main switching devices,
each auxiliary circuit including a half-bridge connection of auxiliary
switching devices with the junction therebetween coupled to the junction
between the main switching devices of the corresponding half-bridge. Under
heavy load conditions, sufficient energy is stored in the main resonant
inductor to commutate the junctions joining the main switching devices in
the resonant converter, resulting in zero-voltage switching for the main
switching devices. Under light load conditions, a phase shift is
introduced between the corresponding main and auxiliary switching devices,
and the auxiliary resonant inductor currents are increased to a level
sufficient for the sum of the main resonant inductor current and the
corresponding auxiliary resonant inductor current to provide zero-voltage
switching for all the bridge switching devices.
Inventors:
|
Schutten; Michael J. (Schenectady, NY);
Vlatkovic; Vlatko (Niskayuna, NY);
Steigerwald; Robert L. (Burnt Hills, NY)
|
Assignee:
|
General Electric Company (Schenectady, NY)
|
Appl. No.:
|
506312 |
Filed:
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July 24, 1995 |
Current U.S. Class: |
363/17; 363/98; 363/132 |
Intern'l Class: |
H02M 003/335 |
Field of Search: |
363/17,95,98,131,132
|
References Cited
U.S. Patent Documents
4916599 | Apr., 1990 | Traxler et al. | 363/65.
|
4992919 | Feb., 1991 | Lee et al. | 363/17.
|
5267138 | Nov., 1993 | Shores | 363/98.
|
5479337 | Dec., 1995 | Voigt | 363/131.
|
Primary Examiner: Wong; Peter S.
Assistant Examiner: Vu; Bao Q.
Attorney, Agent or Firm: Breedlove; Jill M., Snyder; Marvin
Claims
What is claimed is:
1. A power converter, comprising:
a resonant converter comprising a bridge connection of main switching
devices and a resonant circuit comprising a main resonant inductance and a
main resonant capacitance coupled to said main switching devices; and
an auxiliary circuit corresponding to each half-bridge of said bridge
connection of main switching devices, each of said auxiliary circuits
comprising a half-bridge connection of auxiliary switching devices with a
junction therebetween coupled to the junction between the main switching
devices of the corresponding half-bridge, each of said auxiliary circuits
further comprising an auxiliary resonant inductance coupled between the
junction between the auxiliary switching devices and the junction between
the main switching devices of the corresponding half-bridge.
2. The power converter of claim 1 wherein said bridge connection of main
switching devices comprises a full-bridge connection of said main
switching devices.
3. The power converter of claim 1 wherein said bridge connection of main
switching devices comprises a half-bridge connection of said main
switching devices with a junction therebetween.
4. The power converter of claim 1 wherein said resonant converter comprises
a series resonant converter with said main resonant inductance and said
main resonant capacitance connected in series.
5. The power converter of claim 1 wherein said resonant converter comprises
a parallel resonant converter with said main resonant inductance and said
main resonant capacitance connected in parallel.
6. The power converter of claim 1 wherein said resonant converter comprises
a series/parallel resonant converter comprising a combination of series
and parallel connections of said main resonant inductance and said main
resonant capacitance.
7. The power converter of claim 1 wherein current in said auxiliary
resonant inductors is controlled to provide zero-voltage switching.
8. The power converter of claim 7 wherein the current in said auxiliary
resonant inductors is controlled by controlling a phase shift between the
corresponding main and auxiliary switching devices.
Description
FIELD OF THE INVENTION
The present invention relates generally to power converters and, more
particularly, to a resonant converter with auxiliary circuitry for
achieving high efficiency operation over a wide load range.
BACKGROUND OF THE INVENTION
Loss of zero-voltage switching capability (i.e., switching active switching
devices with substantially zero voltage thereacross) at light load
conditions in a resonant converter (e.g., a series resonant converter or a
phaseshifted series resonant converter) leads to a significant increase in
switching losses, excessive generation of electromagnetic interference
(EMI), and problems in controlling output voltage. These problems are
typically solved by adding one or more reactive components to the resonant
circuit for storing some reactive energy. Disadvantageously, however, such
an approach significantly changes the control characteristics of the
converter such that the converter control is very complex and results in a
loss of output voltage control. Furthermore, the additional reactive
energy is circulated through the converter at heavy loads, leading to an
increase in conduction losses and a reduction in efficiency. Many systems
in which a resonant converter would be very useful are intolerant of high
EMI and excessive heat generation, and require very precise voltage
control under wide load variations. Accordingly, it is desirable to
provide a resonant converter capable of providing low EMI, tight output
voltage control, and high efficiency over a wide load range.
SUMMARY OF THE INVENTION
In accordance with the present invention, low-power auxiliary circuitry is
added to a resonant converter for providing high efficiency operation, low
EMI, and fight output voltage control over a wide load range. The resonant
converter comprises a full-bridge or half-bridge connection of main
switching devices, each switching device having a snubber capacitor and an
antiparallel diode coupled thereacross, and a resonant circuit coupled to
the main switching devices. There is an auxiliary circuit corresponding to
each half-bridge connection of main switching devices, each auxiliary
circuit comprising a half-bridge connection of auxiliary switching devices
with the junction theretween coupled through an auxiliary resonant
inductance to the junction between the main switching devices of the
corresponding half-bridge. Under heavy load conditions, sufficient energy
is stored in the-main resonant inductor to commutate the junctions joining
the main switching devices in the resonant converter, resulting in
zero-voltage switching (ZVS) for the main switching devices. Under light
load conditions, a phase shift is introduced between the corresponding
main and auxiliary switching devices, and the auxiliary resonant inductor
currents are increased to a level sufficient for the sum of the main
resonant inductor current and the corresponding auxiliary resonant
inductor current to provide ZVS for all the bridge switching devices.
Advantageously, therefore, the resonant converter operates with high
efficiency and low EMI over a wide load range, and allows phase-shifted
pulse width modulation and frequency control over a wide load range
without loss of ZVS capability. As an additional advantage, the control
characteristics of the resonant converter are unaltered.
BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the present invention will become apparent
from the following detailed description of the invention when read with
the accompanying drawings in which:
FIG. 1 schematically illustrates a resonant converter, including a series
resonant circuit, with auxiliary circuitry in accordance with the present
invention;
FIGS. 2A and 2B schematically illustrate a series/parallel resonant circuit
and a parallel resonant circuit, respectively, which may be used as
alternatives to the series resonant circuit of FIG. 1 in accordance with
the present invention;
FIG. 3 graphically illustrates gate drive signals and voltage and current
waveforms for the series resonant converter of FIG. 1 operating under
heavy load conditions;
FIG. 4 graphically illustrates gate drive signals and voltage and current
waveforms for the series resonant converter of FIG. 1 operating under
light load conditions;
FIG. 5 illustrates simulation results for the converter of FIG. 1 operating
at an input voltage of 400 V, an output voltage of 80 kV, and an output
current of 1250 mA;
FIG. 6 illustrates simulation results for the converter of FIG. 1 operating
at an input voltage of 400 V, an output voltage of 150 kV, and an output
current of 10 mA; and
FIG. 7 schematically illustrates an alternative embodiment of a resonant
converter, including a half-bridge connection of main switching devices,
with auxiliary circuitry in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates a resonant converter 10 with auxiliary circuits 12 and
14 according to the present invention. The resonant converter 10 comprises
a conventional resonant converter 11 comprising main switching devices Q1,
Q2, Q3 and Q4 connected as shown in a full-bridge configuration, each
switch having a snubber capacitor C1-C4, respectively, and an antiparallel
diode (which may be an integral device) D1-D4, respectively, coupled
thereacross. The resonant converter 11 comprises a resonant circuit 13,
which is illustrated in FIG. 1 as a series resonant circuit with a
resonant capacitor Cr and a resonant inductor Lr connected in series with
a load (coupled through transformer T) between junctions a and b joining
the switching devices of each half-bridge, Q1 and Q4, and Q3 and Q2,
respectively. The load circuit is connected to the secondary winding of
transformer T at points c and d, as shown, and comprises a parallel
connection of a full-wave rectifier 16, a filter capacitor Cf, and a load
18.
A series resonant converter is illustrated in FIG. 1 by way of example only
since the principles of the present invention apply equally to other types
of resonant circuits, including, for example, a series/parallel resonant
circuit and a parallel resonant circuit, as illustrated in FIGS. 2A and
2B, respectively.
There are basically two conventional resonant converter control methods.
The more common of the two is switching frequency control. The problem
with switching frequency control is that, at light loads, output voltage
control can only be maintained if the switching frequency is increased to
very high values (which is usually not practicable in the series resonant
converter of FIG. 1).
The second conventional type of control useful in a full-bridge resonant
converter is phase-shifted pulse width modulation (PSPWM) wherein the
switching frequency is fixed, but the phase shift between the two
half-bridges is varied. In this way, the pulse width of the voltage
between junctions a and b is modified to effectively provide output
voltage control by PWM. Using this control method, the output voltage
control is theoretically maintained even at a no-load condition by
operating both half-bridges in phase (i.e., Q1 and Q3 switch
simultaneously, and Q4 and Q2 switch simultaneously, so that v(a,b)=0).
The main problem of PSPWM control is loss of zero-voltage switching (ZVS)
at light loads. When ZVS is lost, switching losses increase and excessive
EMI is generated. In addition, the control characteristics of the
converter are influenced by parasitic oscillations caused by hard
switching.
In accordance with the present invention, the problems of the conventional
resonant converter control methods, described hereinabove, are overcome by
adding auxiliary circuitry. In particular, for the full-bridge resonant
converter of FIG. 1, two auxiliary circuits 12 and 14 are added. Each
auxiliary circuit comprises a half-bridge connection of low-current
auxiliary switching devices Qx1-Qx4 and Qx3-Qx2, respectively; the
junction between the switching devices of each auxiliary half-bridge is
connected through an auxiliary inductor Lx1 and Lx2, respectively, to a
corresponding one of the junctions a and b, respectively, between the main
switching devices Q1-Q4 and Q3-Q2, respectively, of the series resonant
converter. Auxiliary diodes DX1, DX2, DX3 and DX4 are shown as being
coupled in an antiparallel relationship with auxiliary switching devices
QX1, QX2, QX3 and QX4, respectively. Advantageously, the current rating of
the auxiliary components is typically significantly less than the current
rating (e.g., less than 10%) of the main switching devices and resonant
inductor in the series resonant converter.
FIG. 3 illustrates gate drive signals for switching devices Q1-Q4 and
Qx1-Qx4 and converter waveforms for a series resonant converter with
auxiliary circuitry in accordance with the present invention, such as that
of FIG. 1, under heavy load conditions. The main bridge voltage v(a,b) and
the resonant inductor current waveforms are the same as for a conventional
series resonant converter. At heavy loads, there is plenty of energy
stored in the resonant inductor Lr, and this energy is used to commutate
nodes a and b in resonant fashion, thus providing ZVS operation for
switching devices Q1-Q4. The auxiliary inductor currents ix1 and ix2 are
controlled by controlling the phase shifts between the corresponding main
and auxiliary switching devices, i.e., the phase shifts between Q1 and
Qx1, Q2 and Qx2, Q3 and Qx3, and Q4 and Qx4, respectively. If, for
example, there is no phase shift between the main and auxiliary switching
devices, the voltages v(a,ax) and v(b,bx) are zero so that ix1=ix2=0.
Under heavy load conditions, the auxiliary inductor currents ix1 and ix2
are kept at a very low level, typically below 2% of the resonant inductor
current and are sufficient only to commutate the nodes ax and bx and
provide ZVS conditions for the auxiliary switching devices.
FIG. 4 illustrates gate drive signals for switching devices Q1-Q4 and
Qx1-Qx4 and converter waveforms for a series resonant converter with
auxiliary circuits in accordance with the present invention, such as that
of FIG. 1, under light load conditions. Under light load conditions,
operation of the main series resonant converter bridge is essentially the
same as conventional phase-shifted PWM. At light loads, the current iLR is
not sufficient to commutate nodes a and b. For that reason, the phase
shift between the corresponding main and auxiliary switching devices
(i.e., v(ax,a) and v(bx,b)) is introduced, as illustrated, and the
currents ix1 and ix2 are increased to a level sufficient for the sum of
the currents iLR and ix1 (or ix2) to provide ZVS for all the bridge
switching devices.
FIG. 5 illustrates simulation results for the converter of FIG. 1 operating
at an input voltage of 400 V, an output voltage of 80 kV, and an output
current of 1250 mA; and FIG. 6 illustrates simulation results for the
converter of FIG. 1 operating at an input voltage of 400 V, an output
voltage of 150 kV, and an output current of 10 mA. For these simulation
results, the parameters of a high voltage transformer were used for
transformer T, including leakage inductance and winding capacitance.
FIG. 7 illustrates an alternative embodiment of a resonant converter
including auxiliary circuitry according to the present invention
comprising a half-bridge connection of main switching devices Q1 and Q4
and a single auxiliary half-bridge circuit with auxiliary switching
devices Qx1 and Qx4. The output of the half-bridge resonant converter is
controlled by frequency control, and the phase between the auxiliary
switching devices and the main switching devices is varied in order to
maintain ZVS at light loads.
Advantageously, a resonant converter with auxiliary circuitry in accordance
with the present invention operates with high efficiency and low EMI over
a wide load range, and allows phase-shifted pulse width modulation and
frequency control over a wide load range without loss of ZVS capability.
As an additional advantage, the control characteristics of the resonant
converter are virtually unchanged.
While the preferred embodiments of the present invention have been shown
and described herein, it will be obvious that such embodiments are
provided by way of example only. Numerous variations, changes and
substitutions will occur to those of skill in the art without departing
from the invention herein. Accordingly, it is intended that the invention
be limited only by the spirit and scope of the appended claims.
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